Aerospace electric power generators, such as those utilized on a commercial aircraft, are typically wound-field synchronous machines, and can deliver highly regulated electrical power over a wide range of operating speeds and loads. Wound-field synchronous machines are required to meet stringent power quality standards that include, but are not limited to, maximum allowable instantaneous voltage distortions induced by non-linear loads, peak-to-peak phase current during electric start with pulse-width modulation inverters to prevent unwanted high peak phase current, and phase over-voltages encountered during load removals. These conditions are particularly exacerbated at high operating speeds and power levels. In order for the wound-field synchronous machine to achieve acceptable instantaneous transient behavior, the wound-field synchronous machine's sub-transient inductance is bounded within an appropriate range that covers the entire operating speed and load conditions of the specific application.
In conventional wound-field design, sub-transient inductance is an important characteristic that is incorporated into the wound-field synchronous machine by embedding a damper cage in the rotor surface. The damper cage includes damper bars that are displaced along, and embedded in, the surface of the rotor poles of the wound-field synchronous machine and brazed on both ends to form an amortisseur circuit. Typically, the damper bars are equally spaced on each pole surface such that the spacing between damper bars is close to the stator tooth pitch, and the total angular span of each amortisseur circuit is strictly limited to the width of the rotor pole body due to the physical form factor of the rotor pole. As a result of the strict limitations on the location of the damper bars, a limited number of damper bars can be placed within the pole body of the rotor. This strict limitation restricts the range, or minimum value, of the sub-transient inductance that can be achieved in the wound-field synchronous machine.